The present disclosure relates generally to microchips for collecting particulates and particulate fractional collection apparatuses provided with the microchips. More particularly, the disclosure relates to the microchip and the technology for fractionally collecting particulates subject to collection such as cells and micro-beads from a solution in which multispecies particulates are admixed.
Flow cytometry is an analytical method for examining and deciding the kind, size, structure, and so forth, of each of particulates which are flowing through channel in single line, by irradiating the particulates with excitation light such as laser light at a specific wavelength and by detecting the light emitted from the particulates, such as fluorescence and/or scattered light. In addition, based on the detection results obtained as mentioned above, it becomes feasible with the flow cytometry to fractionate intended particulates quickly and reliably even if multispecies particulates are contained in a sample liquid, by separating the intended particulates from other particulates based on the detection results obtained as mentioned above, and by collecting the particulates (for example, see “FLOW CYTOMETRY AT WILL” (Second Ed.) supervised by Hiromitsu Nakauchi, as Experiment Protocol Series of Cell Engineering Supplement, Shujunsha Co., Ltd., Aug. 31, 2006).
As the method of the fractional collection, there are utilized in general the charged droplet method configured to electrically charge droplets containing particulates, and the cell capture method configured to collect particulates with water streams using tubes. However, apparatuses adapted to these methods tend to be large in size and cost, and this gives rise to a problem of less versatility. In addition, in the fractionating method of forming droplets such as the charged droplet method, since the mechanism of droplet formation is sensitive to physical properties of liquid such as surface tension and viscosity, there encountered is another problem of causing the variation in the frequency of droplet formation and in the size of the droplets, affected by the change of measurement conditions.
Furthermore, in the fractionating method of forming droplets, since foreign matters are deposited on an ejection nozzle, thereby possibly varying the direction of droplet ejection, it is necessary to operate the system while readjusted frequently by a skilled worker. In addition, since incidental satellite (mist) generation inevitably occurs even if the droplet formation system is operating stably, that may result in not only failing the collection of intended cells, but also scattering cells in the surroundings.
Consequently, the methods of using microchips have been proposed recently, in which the microchips are each provided with minute channels formed in a substrate of either inorganic material such as silicone, glass, and so forth; or polymer material such as plastic and so forth (see the undermentioned Patent Documents 1 through 6, for example). As an example, WO 2005/121767, as Patent Document 1, discloses the technique of guiding a sample utilizing dielectrophoretic force so that the sample flowing through a main flow path in a microfluidic device is guided to a predetermined flow path. In the analytical fractionating apparatus disclosed in this Patent Document 1, the dielectrophoretic force is generated by providing multiple electrodes around the circumference of the main flow path in the microfluidic device and applying ac voltages to the electrodes.
On the other hand, Japanese Unexamined Patent Application Publications No. 2003-274924 and 2004-113223, as Patent Documents 2 and 3, respectively, disclose the technique of guiding cells to a predetermined branch flow path using an electroosmotic pump provided inside a microchip. In the cell separation apparatus disclosed in each of Patent Documents 2 and 3, the electroosmotic pump is provided on the chip, and intended cells are guided to a specific flow path by operating the electroosmotic pump. In addition, in Japanese Unexamined Patent Application Publications No. 2006-29824 and 2007-330201, as Patent Documents 4 and 5, respectively, the technology is disclosed of moving desired cells to a channel for cell fractionation with an optical tweezers utilizing laser light.
Furthermore, in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-538727, as Patent Document 6, the technology is disclosed of leading particulates to a predetermined branch flow path using an actuator. FIGS. 1A and 1B are sectional views schematically illustrating the operation in sequence of the steps carried out in the microfluidic system described in Patent Document 6. As shown in FIGS. 1A and 1B, in the microfluidic system described in Patent Document 6, a pair of sealed chambers, 102a and 102b, are provided adjacent to a flow path 101. The sealed chambers 102a and 102b are each connected to the flow, path 101 at a location immediately before junction point 101a through side paths 103a and 103b. In addition, a meniscus is formed in each of the side paths 103a and 103b with some of the liquid flowing into the side paths out of the liquid flowing through the flow channel 101.
When fractionating a particulate 104a by the microfluidic system, as shown in FIG. 1A, the sealed chamber 102a is pressed by an actuator 105 according to the timing of the particulate 104a arriving at the location leading to the side paths 103a and 103b. The liquid in the side path 103a is thereby pushed out toward the flow path 101, the flowing position of the particulate 104a is deflected in the direction of the side path 103b, and also the meniscus in the side path 103b is displaced in the direction of the sealed chamber 102b. 
Subsequently, as shown in FIG. 1B, after the particulate 104a flows through passing the location connected to the side paths 103a and 103b, the pressing force by the actuator 105 is released, and the location of the meniscus in each of the side paths 103a and 103b is brought back to the previous location. As a result, the particulates 104b other than those subjected to collection, are made to flow through in the middle of the flow channel 101 and into a branch channel 106 subsequently, while only the particulates 104a subjected to the collection can be led to flow into a branch channel 107.